Cars

Metalworking Boot Camp Part 2

The Fundamentals of Metalshaping: Stretching, Shrinking, and Bending

In the first article of this series we gave an overview of metallurgy and looked at the most fundamental tools for doing metalwork: hammers and dollies. This time we’ll show you how to use these simple tools, and a few more, to move metal with precise control.

The metals used to make cars and trucks (primarily steel and aluminum) are comprised of crystals, joined by strong bonds. Fortunately for us, these bonds are somewhat elastic, which gives these metals their superb blend of strength and workability.

If you break all metalshaping processes down to the fundamentals, there are only three ways to change the shape of sheetmetal: shrinking, stretching, and bending. Let’s look at each of these in detail.

1. Stretching. Metals can’t change volume very much, so if you make them thinner, they get wider. This is similar to rolling out a ball of dough to make a pizza crust. The more you roll the dough, the thinner and wider it gets. Metal works in a similar way. You can stretch metal by hammering on-dolly, which we’ll cover shortly. There are a number of machines that stretch metal, too. If you take a flat piece of sheetmetal and thin the entire piece uniformly, it will stay flat but grow wider. The magic happens when you stretch the center more than the edges, which causes the metal to bulge, or “dome.” This principle allows us to put nearly any shape we want in metal, even with simple tools.

2. Shrinking. If a panel is domed more than you want, you can reduce the amount of doming by shrinking the metal. There are several ways to do this, but the simplest is hammering off-dolly. This rearranges the grains of the metal and actually makes the metal thicker. You can also shrink metal with heat.

3. Bending. This is a very versatile method for changing the shape of a piece of sheetmetal without changing the thickness. In some cases you can bend metal by hand, but more often it is bent by hammering, or with machines like a bending brake or slip roll.

Now let’s see how these principles are applied for shaping metal. Hammers and dollies are the basic tools for doing bodywork and metal shaping, and we’ll look closely at using them to move metal with precision. To stretch metal, a dolly block is held under the metal, and the metal is hit with a hammer from the top, so a small area of metal gets squeezed between the face of the hammer and the dolly. This is the definition of hammering “on-dolly.” As long as the hammer is swung with sufficient force, it will cause the grains of metal in the contact patch to move—making that spot of metal microscopically thinner and a tiny bit wider. If you are working on a metal panel that is domed, the area that you stretch will dome up slightly more.

If you start with a flat sheet and stretch any area, the metal doesn’t necessarily “know” which way to go, and the direction it moves will be controlled by several variables. If the metal moves in the “wrong” direction, it’s easy to change the direction when the panel is nearly flat by simply applying some force in the direction you want. Once the panel gains some shape, it’s much more difficult to change the direction of the doming.

The shape of the hammer and dolly are important when hammering on-dolly. You usually get the best results from using a dolly that is close to the curvature of the panel you’re working, and using a hammer with a low-crown face. I use a hammer with a 12-inch radius face for most of my work, and my favorite “egg” dolly has faces that will work on anything from a flat piece of metal right up to something domed as much as a basketball.

To hammer off-dolly, the dolly is held beneath the metal being worked, and the hammer strikes the top surface—but in this case, the hammer contacts the metal AWAY from where the dolly is touching. This process is distinctly different from hammering on-dolly, and there are several ways this technique can be used. One of the easiest is putting a curl on the edge of a piece of metal. First, you select a dolly with the curvature you want the metal to have, and you hold the dolly inboard from the edge of the panel. Then, you strike the edge of the metal with the hammer, which causes the metal to “curl” down around the dolly. This technique bends the metal more than it stretches or shrinks it, and it can be used for small, medium, or large radius curves.

Another use of the off-dolly technique is to reduce the curvature of a panel, or to bring high spots down. Imagine a fender that has been dented by a tire hurling a stone against it. This type of damage can be repaired by holding the dolly so it touches the metal inside the fender just a slight distance away from the dent and striking the top of the dent with the hammer. It may be hard to visualize but this process shrinks the metal, making it microscopically thicker!

When hammering off-dolly, if the dolly is touching an inch or less away from the hammer strike, pressing up on the dolly causes that area to move up when the hammer hits. This is very useful for straightening “lumpy” metal, with many high and low spots. If the dolly is touching more than an inch from the hammer strike, the metal hit with the hammer will still move down, but you lose the benefit of the lifting action of the dolly.

On high-crown panels, you can often make a high spot go down by simply hitting it with a hammer. The more crown a panel has, the stiffer it is, so the strength of high-crown panels will hold the panel in place while you tap any high spots down. This doesn’t work on a low-crown panel like a doorskin, so that’s where you’ll need to use the off-dolly technique, or heat shrinking.

For shrinking steel panels, you need to heat them until they develop some color. I normally use an oxy-acetylene torch for this, but a propane torch will work, too. I usually heat a spot about the size of a penny or quarter.

You can gauge the temperature of a steel panel by its color. At 450 degrees F, steel turns a “straw” color. At 550 degrees F, it turns blue. At 1,300 degrees F, it turns a bright cherry red. The higher the heat, and the larger the spot that’s heated, the more shrinking you get. Shrinking happens very quickly, and it’s easy to overdo it, so I recommend starting very conservatively.

You will get some shrinking just by heating a small spot and letting it cool naturally. This is because metal expands as it’s heated, so the heated area is pushing outward against the cool metal surrounding it. This effectively creates a constricting band around the hot spot, and once you heat the metal enough to soften it—even slightly—these constricting forces cause the hot spot to shrink a little bit.

You can accentuate the shrinking by hammering off-dolly on the spot that bulges up as you heat it. Sheetmetal cools very quickly, so you need to move fast to maximize the effect. Be extremely careful not to hit on-dolly when the metal is hot or you will quickly overstretch it. Some people use an aluminum hammer for shrinking, which lessens the chance of a stray hit on-dolly stretching the metal.

After shrinking, many people quench the metal with water or a blast of compressed air, but I don’t recommend this practice since rapid cooling will harden the metal. This makes it more difficult to file, or work with, a hammer and dolly. It you let the metal cool naturally the spot you shrank will stay nice and soft, easing any further work needed in that area. If your first shrink doesn’t move the metal enough, you can repeat the process as often as necessary, but it’s best to move around and avoid shrinking any spot more than once.

There are many tools designed to shrink and stretch metal. The English wheel does basically what hammering on-dolly does: it squeezes the metal between the top and bottom wheel, making it slightly thinner and wider. This is how the English wheel causes panels to dome. It’s much easier to work large panels with an English wheel, and a skilled operator can make panels that are glass smooth. The principles, however, are identical to hammering on-dolly.

There are mechanical shrinking and stretching machines, too. Entry-level machines are affordable and they give you a lot of “bang for the buck.” The least-expensive machines have a 1-inch throat depth, but are still very helpful for many projects. There are professional-grade machines that have a 4- to 6-inch throat depth or more.

A good way to make a deep shape, like you might need for a cab corner, is pulling the edges down with a shrinker and doming the center by hammering on-dolly or by using an English wheel. This is much more efficient than shaping the entire panel by shrinking or stretching alone.

Next time, we’ll take a look at a broad variety of ways to cut metal. You can also check out my YouTube channel for a lot of free, in-depth instruction on metalworking and welding.

These are the most basic tools for doing bodywork, the hammer and dolly. As you will see, you can move metal with precision with these versatile tools.
For our first demonstration, we’ll strike “on-dolly” about 20 times. The small “X” is the target I’m aiming for to keep all the blows in a small area.
The straightedge shows how much the metal has been raised by hammering on-dolly. This stretches the metal, causing it to dome up.
Here’s another variation of the hammer on-dolly process. I’m hammering from the inside of the panel, backing the blows up with a forming head that’s held in a vise. I’ve drawn a grid on the panel to keep my hammer blows uniformly spaced.
You can see how much the metal has domed with two passes over the grid, one with hits in the center of the squares and one with the hits at the line intersections.
With two more rounds of hammering, you can see how I have accentuated the doming along the length of the panel. You can actually make panels like doorskins using this simple technique!
I’m using an English wheel here on a flat piece of metal. This is essentially a rolling “hammer on-dolly” device since it stretches metal the same way that hammering does. The shaping goes a lot faster with an English wheel, and it can produce a much smoother panel.
With just a couple of minutes of wheeling, the once-flat panel has domed a lot. If you want more doming, you simply continue wheeling until the shape is as deep as you want.
We’ll take this dent out by hammering off-dolly. Note that the dolly is only touching at the edge of the dent, and the hammer is poised to strike the dent in the center.
Here’s how the dent looks after just a few hammer blows. The hammering is confined to the dent, and the dolly is moved around the perimeter.
After hammering for less than a minute, virtually every trace of the dent has been removed. Hammering off-dolly moves the metal down and actually shrinks it.
Now I’ll demonstrate heat shrinking. I put a large bulge in the panel by hammering from the underside, causing a lot of stretching in the impact zone.
I’m using a propane torch here to heat the metal to a dull blue color, which is about 550 degrees F.
Working quickly while the metal is still hot, I’m hammering off-dolly to shrink the bulged metal down. I do NOT recommend quenching with water or compressed air, which can harden the metal
After letting the panel cool naturally, I’m checking the area with a straightedge. I shrank this a little too much, creating a flat spot. I will have to do some on-dolly hammering to bring this up to the proper contour.
Here’s a metal shrinking machine, which is nearly indispensable for doing metalwork. This small piece of metal has been domed by shrinking the edges. The yellow machine in the background is a stretcher.
Another common technique for shaping metal is bending. There are many ways to do this, but sometimes it can be as easy as using your arms and body to push the panel against a workbench.
Another way to bend metal is to hammer it over a curved form. In this case I’m using a “T” dolly, held in a vise. You can make both large and small curves in this way.
Show More

Leave a Reply

Your email address will not be published. Required fields are marked *

Back to top button
close
Thanks !

Thanks for sharing this, you are awesome !

Close